Abstract classes in Delphi

Can. The point is that you will never have an instance of a base abstract class. There will be an instance of a specific child class in which this method is exactly overridden.

This design pattern, by the way, is called the Template method: in the base abstract class we collect some algorithm from the abstract methods. And the work of this algorithm will depend on the child specific classes.

Example: TCocktail base class defines a cocktail preparation procedure. Child classes implement the specific steps of this process.

Base class:

type TCocktail = class protected procedure AddFirstIngridient(); virtual; abstract; //Добавление первого ингридиента procedure AddSeconsIngridient(); virtual; abstract; //Добавление второго ингридиента procedure AddDecorations(); virtual; abstract; //Добавление украшений function GetName(): string; virtual; abstract; //Название public procedure Prepare; //Алгоритм приготовления коктейля end; { TCocktail } procedure TCocktail.Prepare; begin writeln('Готовим коктейль ' + GetName()); AddFirstIngridient(); AddSeconsIngridient(); AddDecorations(); writeln('Ваш коктейль готов'); end; 

First cocktail

 type TBloodyMary = class (TCocktail) protected procedure AddFirstIngridient();override; procedure AddSeconsIngridient();override; procedure AddDecorations();override; function GetName(): string;override; end; { TBloodyMary } procedure TBloodyMary.AddDecorations; begin writeln('Добавляем лед'); end; procedure TBloodyMary.AddFirstIngridient; begin writeln('50мл водки'); end; procedure TBloodyMary.AddSeconsIngridient; begin writeln('150мл томатного сока'); end; function TBloodyMary.GetName: string; begin result := 'Кровавая Мэри'; end; 

Second cocktail

 type TIrishCoffee = class (TCocktail) protected procedure AddFirstIngridient();override; procedure AddSeconsIngridient();override; procedure AddDecorations();override; function GetName(): string;override; end; { TBloodyMary } procedure TIrishCoffee.AddDecorations; begin writeln('50 сливки (33%)'); end; procedure TIrishCoffee.AddFirstIngridient; begin writeln('50мл ирландского виски'); end; procedure TIrishCoffee.AddSeconsIngridient; begin writeln('80мл кофе'); end; function TIrishCoffee.GetName: string; begin result := 'Ирландский кофе'; end; 

Well, the code itself:

 var cocktail: TCocktail; begin cocktail := TBloodyMary.Create; cocktail.Prepare; cocktail := TIrishCoffee.Create; cocktail.Prepare; readln; end.